In magnetic recording media, such as hard disks for computers, a transducer is positioned adjacent to a magnetic recording media. In longitudinal recording, the transducer records data by magnetizing the magnetic recording media in a longitudinal direction. In other words, the transducer magnetizes parallel to the direction of relative movement between the magnetic medium and the transducer. In perpendicular recording, the transducer records data by magnetizing the magnetic recording media in a perpendicular direction.
Computers manipulate information using binary symbols that include the alphabet {1,0}. When writing to the magnetic media, computers magnetize the magnetic media in opposite magnetic directions. To more accurately reflect the bipolar physical magnetization, the information signals are represented by the alphabet {1,−1}.
Each bit storage location on a disk drive is a magnetic domain that includes a number of grains, which are crystals of magnetic material. Perpendicular recording allows a smaller grain size than longitudinal recording before encountering a superparamagnetic effect. Perpendicular recording allows tracks that are more narrow and well defined than those in longitudinal recording. In other words, perpendicular recording allows significantly higher areal densities.
The write signals in longitudinal and perpendicular recording are also different. Referring now to FIG. 1, a write signal 10 in longitudinal recording is normally at a DC null 12. The write signal 10 transitions from the DC null 12 to +a or −a during a transition 14 and returns to the DC null 12 until a subsequent transition occurs. In perpendicular recording, however, a write signal 16 transitions between +a or −a. In other words, the write signal 16 does not return to the DC null 12.
In perpendicular recording, a preamplifier and input AC coupling introduces DC distortion. The distortion is monitored and compensation is provided. As the number of consecutive a's or −a's increases, a DC offset of the perpendicular recorder tends to drift. In addition, the number of alternating transitions between adjacent symbols should be limited to the extent possible.